The present invention relates to a process for recycling electronic scrap material such as optical recording media, for instance CDs and DVDs. In part the process involves delamination of electronic scrap material in order to recover the constituent components of it.
The use of optical recording media such as CDs and DVDs has increased enormously in recent years with applications in the computer and audio/visual entertainment industries. These media typically include a metal, such as aluminium and/or precious metals (typically gold or a mixture containing gold) provided on a polymeric substrate such as a polycarbonate. Typically the metal is provided on the polymeric substrate as a thin metallic film. A protective layer is usually formed on the outer surfaces, and acrylates such as polymethyl (meth)acrylates are commonly used in this respect. These optical recording media have a relatively short life of only a few years as they may become superseded or outdated. This leads to a considerable waste stream. Furthermore, there is a waste stream generated at the time of manufacture due to strict quality control standards: typically up to 20% of manufactured product is rejected. Disposal of such waste is an increasing concern and recycling techniques are being investigated. Similarly other electronic scrap materials including precious metals and plastics are becoming increasingly available, and disposal of them is an important consideration also. In this specification the term xe2x80x9celectronic scrap materialxe2x80x9d is used to embrace all such materials, including optical recording media such as CDs and DVDs.
Known processes for recycling electronic scrap material include smelting and chemical dissolution treatments that are primarily directed at recovering precious metals. Such methods tend to be damaging to any associated plastic material and, furthermore, the disposal of the plastic material can lead to environmental problems. Accordingly, chemical recycling methods tend not to be sympathetic to the environment.
U.S. Pat. No. 5,306,349 assigned to Sony Music Entertainment Inc discloses a method for removing lacquer and aluminium coatings from the polycarbonate substrate of compact discs. This method uses an alkaline solution and the application of ultrasonic energy to compact discs immersed in this solution. Such treatments are not satisfactory when gold is present. Furthermore, the treatment could degrade the polycarbonate plastic.
Recycling methods which rely on physical rather than chemical mechanisms have also been applied. For example, attempts have been made to remove a metallic layer from a polymeric substrate by slicing or shaving techniques. However, with such techniques the throughput can be relatively low and thus uneconomic where a large number of articles are to be recycled. Additionally, for security reasons, manufacturers sometimes choose to cut articles to be recycled (e.g. CDs and, DVDs) into pieces prior to transportation to a recycling facility. Techniques such as slicing and shaving cannot be applied practically to the articles in cut form.
With this background in mind, the present invention seeks to provide a process for recycling an electronic scrap material which does not involve hazardous or potentially harmful chemicals and which is thus environmentally friendly, has high throughput and does not rely on the material to be recycled being in unitary form. The method is easy to perform and economic in practice. Moreover, it has been found that the polymer component it is desired to recover does not significantly degrade during the recycling process so that production of a high grade recycled product may be achieved.
Accordingly, the present invention provides a process for recycling an electronic scrap material comprising a metal provided on a polymeric substrate, which method comprises:
milling flaked electronic scrap material with a bead impact material in the presence of water to produce flakes of cleaned polymeric substrate;
adding water to the milled material and separating the flakes of cleaned polymeric substrate from metal-containing material;
dewatering and drying the flakes of cleaned polymeric substrate; and
treating the metal-containing material to recover the metal.
In the present specification the term milling is used to denote any process by which attrition of the surface of the flakes of scrap material by contact with the bead impact material may be achieved. Such attrition removes of surface layer(s) of the scrap material and is a fundamental aspect of the present invention. In the context of the present invention the terms milling and attrition may be used interchangeably in addition to their usual meanings in the particle science industry.
The scrap material to be milled is in flake form. This means that the scrap material, for example a CD or DVD, is cut into individual flakes. This may be achieved using a conventional shredding machine or granulator. Ideally shredding/granulation cuts the material cleanly without any bending or distortion so that the resultant flakes are planar (assuming the original unitary material is planar). Bending or distortion of the scrap material during shredding/granulation can lead to metal smearing of the polymeric substrate material and/or may reduce the efficiency of the subsequent milling operation due to shielding effects.
The scrap material may be cut into flakes at the intended site of recycling or it may be supplied to the site in flake form. Thus, the process of the invention may include as preliminary steps the transportation of electronic scrap material to a flaking station followed by cutting the material into flakes. The flakes may then be transported to a recycling station where subsequent processing is carried out in accordance with the steps of the invention already described. If the scrap material is provided as is to the site where recycling is to take place, flake preparation will obviously be required prior to the subsequent processing. It is envisaged that in practice the scrap material would be supplied to a recycling facility in flake form. Depending on the flake size, further cutting of the flakes may be required prior to processing.
In an embodiment of the invention the flake size prior to milling is usually in the range 1 to 20 mm, for example 1 to 15 mm, preferably 4 to 8 mm and more preferably 4 to 6 mm. The optimum flake size will depend upon such factors as the size of the bead impact material used in the milling step. If the flake size is too small, valuable polymeric material may be lost as fines in downstream process steps.
The flake size has implications with respect to the size of bead impact material used in the milling step. Thus, for relatively large flakes, milling may be optimised using a different size bead impact material when compared with the size of bead impact material most suitable for attrition of smaller flakes.
In a preferred embodiment, the flake size falls within a narrow distribution so that a correspondingly narrow distribution of size of bead impact material may be used in the milling step. For example, it is preferred that at least 50 wt %, for instance at least 75 wt % of flakes have a size of 4-8 mm, and preferably 4-6 mm. It will be appreciated however that a given batch of flakes to be treated may well have a broad distribution of flake size, and to optimise the milling step, bead impact material having a range of particle sizes may be used.
In an embodiment of the invention the flakes to be processed may include a distribution of flake size such that it will be most efficient to mill flakes above a predetermined size with a first size of diameter bead impact material and to mill flakes at or below the predetermined size with a second size diameter of bead impact material. This embodiment of the present invention would thus comprise the following steps:
milling flaked electronic scrap material that is above a predetermined flake size with a first size diameter of bead impact material in the presence of water to produce flakes of cleaned polymeric substrate;
milling flaked electronic scrap material that is at or below the predetermined size with a second size diameter of bead impact material in the presence of water to produce flakes of cleaned polymeric substrate;
adding water to the milled material and separating the flakes of cleaned polymeric substrate from metal-containing material;
dewatering and drying the flakes of cleaned polymeric substrate; and
treating the metal-containing material to recover the metal.
In this embodiment, the predetermined size is typically about 3 mm. The first size of bead impact material is usually greater than 1000 xcexcm and the second size of bead impact material is usually less than 1000 xcexcm. The first attrition or intensive shear stage generally leads to 80-90% removal of metal from the polymeric substrate.
As a further feature of this embodiment, prior to the first milling step, the process may comprise:
transporting a waste stream of electronic scrap material to a flaking station;
dividing the material into flakes at the flaking station; and
transporting the flakes to a milling station.
For the typical flake size contemplated (1 to 20 mm) the milling step may be performed using a bead impact material having a particle size as low as 50 xcexcm. If the flake, or a significant proportion of the flake (for instance in excess of 50 wt %), has a particle size of 20 mm the particle size of the bead impact material may be as high as 3 mm. The function of the bead material is to remove by attrition the metal, and any additional layer(s) overlying the metal, and one skilled in the art would be able to select a suitable bead impact material size, or distribution thereof, based on this intended function and the following description of other relevant operational parameters.
As bead impact material any material serving the intended function of removing the metal, and any additional layer(s) overlying the metal, may be used. Thus, the bead impact material should have a suitable surface hardness. Interactions between individual flakes may also contribute to the attrition process. For optimum effect it is preferred that the bead impact material is in the form of particles having irregular surfaces. It is also preferred that the bead impact material has a roughened rather than smooth surface. As the milling step takes place in water, the material should also be suitably stable in water. The process of the invention will also typically be carried out at elevated temperature and it follows from this that the bead impact material should also have the necessary integrity at the maximum temperature at which the process is performed.
Examples of bead impact material which may be used include plastics, silicon-containing materials, ceramics and metal powders. Examples include pumice, sand, powdered glass, diatomaceous earth and silicon carbide. Such materials are commercially available. It is also possible to make use of commercially available scouring agents, such as Ajax and Jif, which include abrasive particles in a carrier fluid/paste. It is preferred to use pumice having a particle size of about 300 xcexcm or less or silica having a particle size of about 150 xcexcm or less. The use of metals as bead impact material can sometimes lead to discolouration of the polymeric substrate due to smearing. Ceramic materials exhibit excellent abrasive properties although this should be counterbalanced with their tendency to be brittle. In an embodiment of the invention it is possible to use the polymeric material itself, possibly recycled from the process described herein, as the bead impact material. Thus, in CDs and DVDs where the polymeric substrate is s polycarbonate, the bead impact material may be recycled polycarbonate.
In an embodiment of the invention it is preferred that the density of the bead impact material is not too dissimilar (xc2x150%) from that of the polymeric substrate it is desired to recover. The effect of this is that there is enhanced mixing and dispersion of the bead impact material and the flake. If the density of the bead impact material and polymeric substrate is substantially different, partitioning the two may occur resulting in less effective attrition. The size of the bead impact material and the flake may also be matched to enhance mixing and dispersion of the two.
The quantity of bead impact material may vary based on the amount of flake material present, and this will affect the rate of attrition. Thus, where the proportion of bead impact material is high relative to the amount of flake present, the rate of removal of metal, and any overlying layer(s), is correspondingly high. However, the rate of attrition should also be balanced with the rate of wear of the material and/or apparatus that will likely ensue. Typically, the weight ratio of bead impact material to flake is 1:30, for example 1:20. In practice the weight ratio chosen will be influenced by the desired process time which in turn may be influenced by things such as electricity costs.
An important aspect of the invention is that the milling step takes place in the presence of water. The water functions as a lubricant and heat transfer medium as well as aiding transport of the milled products. Without wishing to be limited by the following hypothesis, it is believed that the high intensity attrition due to the bead impact material leads to rapid distortion and heating of the flake at the interface between the various layers present, for example at the metal/polymeric substrate interface, leading to loss of adhesion and delamination. It can also be likened to the cutting of oxidised paint for surfaces with a car polish. Typically, the weight ratio of water to flake is from 1:3 to 3:1 and, preferably, 1:1. If the proportion of water is much in excess of this, the process of the invention works less efficiently.
The process of the invention may be carried out under ambient conditions of temperature and pressure, although it will be appreciated that the milling step will itself cause a temperature increase due to frictional interactions. Typically, for a given process time where lower temperatures are used more bead impact material is required. It is possible to carry out the process at elevated temperature and, in part, this may be achieved by using heated water in the milling step. Indeed, it has been found that elevated temperature either imposed externally, for instance by use of hot water and/or thermal jacketing, or generated in-situ is associated with acceleration of the attrition process. In the case of CDs which have an acrylate outer layer overlying the metal layer, the use of elevated temperature also advantageously lowers the integrity of the acrylate layer thereby facilitating its removal. At elevated temperature it is believed that the acrylate becomes tacky/sticky and this property may lead to enhanced removal of the underlying metal layer when the acrylate layer is itself removed. The process of the invention may therefore be carried out at elevated temperature provided that the polymeric substrate it is desired to recover is not adversely affected. Thus, where the polymeric substrate is a polycarbonate the process may be carried out at a temperature of, for example, up to 120xc2x0 C. (in which case the process must be carried out under pressure). Where the process is carried out at atmospheric pressure the maximum temperature will be 100xc2x0 C. In determining the temperature at which the process is carried out the temperature increase due to the milling operation itself must be accounted for.
Milling takes place in any suitable apparatus which is capable of causing high shear between the flakes and bead impact material. Thus, a high shear mixer, a stirred attrition mill, a peg mill mixer or a bead mill may be used. The duration of milling will vary depending upon such factors as the shear imparted by the mixer, the type and proportion of bead impact material, the flake size and the temperature, and may be determined on a case-by-case basis for optimum results.
Prior to milling, it is preferred that the flakes are washed in an aqueous medium containing a detergent or surfactant suitable to remove any particulate and absorbed contaminants from the flake surface.
Subsequent to milling water is added to the flakes of polymeric substrate material. This effectively washes the flakes to remove any products of attrition and the bead impact material. Preferably, the flake is subjected to shear washing with water. Water used in this step may be recycled to the milling step in order to minimise loss of potentially valuable components.
The flakes of washed polymeric substrate are then separated from metal-containing material, and other materials such as derived from any overlying layer(s), using conventional techniques. The separated flakes of polymeric substrate may then be dewatered and dried by conventional techniques. The polymeric substrate is now in a form which may be useful for further applications. Depending upon the grade of the resulting polymer, this may include re-use in an electronic material such as a CD or DVD. Alternatively, the polymeric substrate material may be used in applications where the grade of the material is not as critical.
The metal-containing material is treated to recover the metal. This may be achieved using a thickener, such as a Lamella thickener, which allows the metal solids to settle and flowover water to be removed. A conventional flocculating agent may be used in this step. The metal may then be recovered by filtration techniques such as by use of a pressure filter, drum filter or belt filter. A cake of metal is obtained.
In the case of CDs and DVDs the scrap electronic material includes primarily polycarbonate polymers, aluminium and gold. However, a number of other materials may also be present. These include various adhesives, polymeric materials, lacquers and printing inks. To maximise the value of the polycarbonate polymer it is important and preferred that in its recovered state it is free from contaminants and also that its molecular weight has not been adversely reduced by the recovery process. Any undesirable reduction in molecular weight may be remedial increasing the molecular weight in accordance with conventional techniques. It is less important to have the gold free from other materials as the weight concentration of the gold is relatively low, typically only 100 ppm.
In an embodiment of the invention the process may be used to recover polycarbonate and metal from a CD or DVD. CDs typically have a substrate layer of polycarbonate which is coated on one side with a metal backing layer (usually aluminium). The CD also includes as an outer layer over the metal backing layer an acrylate layer. The present invention may be applied to strip off the layer of acrylate and the layer of metal in order to isolate the polycarbonate. The metal may also be recovered. DVDs may be single-or double-sided. In the double-sided version a layer of metal, typically gold or a gold alloy, is sandwiched between two substrate layers of polycarbonate and acrylate layers may be provided as outer layers on the respective polycarbonate substrate layers. The metal is usually bonded to the polycarbonate layers with an adhesive. In order to access the metal layer it is necessary to delaminate the DVD structure. Cutting the DVD into flakes usually initiates this and subsequent milling will further it. The polycarbonate may be isolated as described using milling/attrition.
As a practical example when recycling a CD or DVD of the type described it is preferred that the CD or DVD is cut into flakes having an even distribution of particle size of about 6 mm. The weight ratio of water:flake used is typically 1:1 and the process temperature about 80xc2x0 C. As bead impact material silica powder with a maximum particle size of 150 xcexcm or polycarbonate pellets with a particle size of 1 to 2 mm may be used. In either case the weight ratio of the bead impact material to flake is approximately 1:20.
Although readily applicable to what may be regarded as conventional electronic materials, the invention may be applied to any scrap material in which a metal is provided on a polymeric substrate. For example, the invention may be used to recycle aircraft windows and vehicle headlight reflective backings.
The invention will now be illustrated by the following non-limiting examples. In the examples estimates of sample cleanliness were assessed visually with the aid of an optical magnifier.